Micro- and nanotechnologies can be used to fabricate a radiation sensing device. Radiation sensing and in particular infrared radiation sensing can be utilized for nonvisual environment monitoring, remote thermal mapping, thermal imaging, human occupancy sensing, human gesture recognition and human activity monitoring.
Currently, some fabrication methods are known to fabricate micro-thermo-mechanical devices that operate through the bi-material effect (e.g., U.S. Pat. No. 3,896,309). The bi-material effect allows a mechanical reversible deformation of a free-standing structure upon temperature change. The physical phenomenon behind the displacement is a mismatch of coefficients of thermal expansion (CTEs) of the different materials forming the free-standing structure. Such effect is useful for instance for thermal radiation sensing where radiation energy is converted into a mechanical displacement of a microscopic geometry. To realize a bi-material structure, typically two materials are connected to each other, forming a compound in a long, thin, cantilever-like arrangement. For radiation-sensing applications, an array of bi-material actuated micro-devices is deployed, comprising thin-films that are deposited and structured on top of each other, and which are later released to form a free-standing structure.
Currently, some micro-thermo-mechanical devices are known that include a fabrication method which utilizes partial substrate removal and makes sacrificial layers redundant, in order to release the devices to have them free-standing and function accordingly (e.g., Z. Miao et al. in “Uncooled IR imaging using optomechanical detectors”, in Ultramicroscopy 107 (2007) 610-616, or D. Grbovic et al. in “Arrays of SiO2 substrate-free micromechanical uncooled infrared and terahertz detectors”, in JOURNAL OF APPLIED PHYSICS 104, 054508 (2008)).
Currently-known fabrication methods to produce micro-thermo-mechanical devices include the separate patterning of the first and a separate patterning of the second layer. For example the bottom layer is structured first with a first patterning mask and then the second layer is structured with a second patterning mask on top of the first already patterned layer. In such case the alignment of the second patterning mask is critical to the features from the first mask to realize an accurate overlay of the features. The overlay of the structuring patterns (photolithographic masks) can be bound to strict tolerances. Great effort needs to be put in the alignment procedure to achieve sufficiently precise overlay of the first layer on top of the second layer, or vice versa. Typically the structures have dimensions in the micrometer range—for example the critical bi-material region can be of 3 μm width and 100 μm length. In such example a misalignment of a few micrometers would lead to intolerable misalignment of the critical regions and therefore to complete discard of the microfabrication process at that stage.